Optical transmission systems, and in particular those optical systems employing dense wavelength division multiplexing (DWDM), are desirable because these systems provide extremely wide bandwidths for communications channels. Each optical fiber employed in a DWDM transmission system may typically carry a plurality of 16, 40, 80, or more optical channels, or wavelengths, on the optical fiber. An optical fiber may carry signals originating at a transmitter and passed along a transmission line to a receiver. The transmission line may be very long and may include a plurality of relay stations to boost the signal in order to maintain signal strength. At various points along the transmission line, such as at the transmitter, after one or more relay stations and immediately before the receiver, it is important to monitor and measure various characteristics of the optical signals being transmitted.
A transmission line may comprise a number of optical fibers bundled together into an optical cable. Typical prior art techniques are directed toward monitoring the signals carried by each fiber within a cable, and include the use of mechanical switches or relays to perform physical connection and disconnection of each fiber from a monitoring system. For example, it may be desired to monitor the signals carried by a cable immediately after a relay station. A tap may be placed onto the optical cable, supplying the signal carried by each fiber of the cable to a port of a monitoring system. A mechanical or electromechanical switching system within the monitoring system, for example a system of relays or a rotating mirror, creates and breaks optical connections between each port and analysis circuitry within the monitoring system. When it is desired to monitor the signals being carried by a particular fiber, a connection is established between the port connected to that fiber and the analysis circuitry.
The use of mechanical relays or other mechanical devices to establish and break connections in order to perform analysis suffers from several disadvantages. Mechanical and electromechanical relays are relatively expensive and are much slower and less reliable than purely optical techniques would be.
Accordingly, it would be highly advantageous to provide a more reliable and faster technique for monitoring optical signals than can be achieved through the use of mechanical or electromechanical devices to select between fibers comprising an optical cable.